Unidirectional sheet made of a composite

ABSTRACT

The invention concerns a web comprising an intimate mixture of mixed slivers of long reinforcing staple fibres ( 14 ) obtained by stretch-breaking and long thermoplastic matrix fibres ( 15 ), those different fibres being parallel in the mixture.

The subject of the present invention is a unidirectional sheet made of acomposite.

Such a sheet is intended for producing industrial parts of complexshape, in particular having drawn areas, and able to be used in variousfields, such as the aeronautics field or the automobile field.

Composites are expensive to manufacture and are generally reserved forvery high-performance applications. The composite products are often inthe form of wovens, braids or knits. To limit the cost, it has alreadybeen envisioned to use unidirectional sheets of filaments.

In a manner known per se, the filaments are juxtaposed and held parallelto one another, limiting their superposition.

They then undergo a thermoplastic powder coating operation followed bycalandering by passing them through smooth rolls in order to obtain astiffened sheet. Alternatively, it is possible to make the parallelfilaments pass through a bath of molten polymer before expressing theexcess polymer.

The drawback of such a sheet, apart from its high manufacturing cost, isto provide a continuous array of filaments in the longitudinal directionwhich is ill-suited to the deformations involved in the production ofparts of complex shape, and especially of deep-drawn parts.

It is also known, from document EP 0 466 618, to produce hybrid yarnsobtained by mixing discontinuous carbon fibers with thermoplastic matrixfibers for the purpose of obtaining various preforms from wovens,braids, knits or unidirectional sheets.

Such unidirectional sheets are therefore produced from yarns of circularcross section, which have to be warped and then tied togethertransversely using a polymer yarn. This polymer yarn must be of the samekind as that constituting the meltable fraction containing the yarn. Thespacing of the yarns constituting the warp must be sufficient to allowspreading of the fibers after the matrix has melted, while preventinginterstitial voids. This sheet, because of the fact that the array ofyarns must be open, is fragile and not easy to handle. It is ill-suitedto the cutting of preforms for the subsequent production of consolidatedparts. Furthermore, the manufacture of the yarn, then of theunidirectional sheet, are expensive and render and the productill-suited to the economic constraints in various branches of industry.

The object of the invention is to provide a unidirectional sheet made ofa composite, which comprises at least one type of reinforcing fiber, isobtained economically and is intended for the manufacture of complexparts possibly with regions that are deep drawn, while allowing a veryshort processing and production time.

For this purpose, the unidirectional sheet to which the inventionrelates comprises an intimate mixture of mixed tapes consisting of longdiscontinuous reinforcing fibers obtained by cracking and of longthermoplastic matrix fibers, these various fibers being parallel in themixture.

The technology of cracking continuous fibers offers the possibility ofobtaining tapes consisting of discontinuous fibers of variable lengths,for high-modulus and high-strength fibers, such as carbon fibers, glassfibers or para-aramid fibers. Likewise, the thermoplastic polymericfibers may undergo the same conversion or may be obtained by mechanicalchopping through variable lengths and be converted by the conventionalmeans of carding, combing and drawing.

According to one feature of this sheet, the average length of thereinforcing fibers is around 80 mm with a divergence in the lengths ofless than or equal to 35% with respect to this average length.

As regards the lengths of the thermoplastic fibers, these lie within arange varying between 40 and 180 mm.

According to another feature of the invention, the number of traces T ofreinforcing fibers in the final mixture is greater than 300. This makesit possible to obtain excellent intimacy in the mixing of the fibers toobtain a preconsolidated sheet in the shortest possible time interval.

According to one feature of this sheet, the constituent reinforcingfibers of a sheet are chosen from at least one of the families offollowing materials:

-   -   carbon, coming from PAN, pitch or rayon precursors with the        criterion: 190 GPa<E<700 Gpa(E being the tensile modulus);    -   glass of the E, R, S or D type;    -   para-aramid with the criterion: 60 Gpa<E<140 Gpa(E being the        tensile modulus);    -   polybenzimidazole (PBO) of the AS-H-M type with the criterion:        E>190 Gpa (E being the tensile modulus).

According to one option, the constituent matrix fibers of a sheet arechosen from at least one of the families of the following heat-stablematerials: polyphenylenesulfide (PPS), polyetherimide (PEI),polyethersulfone (PES), polyetheretherketone (PEEK) and polyetherketone(PEK).

According to another option, the constituent matrix fibers of a sheetare chosen from at least one of the families of the following standardthermoplastics: polyether terephthalate (PET), polyamide (PA),polypropylene (PP), polycarbonate (PC), polybutadiene terephthalate(PBT) and polyethylene (PE).

To optimize the cohesion of the sheet, it comprises a tape or yarn ofcomposition identical to the mixed tapes forming the longitudinaldirection, placed in a direction approximately perpendicular to thereinforcing fibers and, for example, being deposited in a zigzag on thesheet.

A process for manufacturing this sheet consists:

-   -   in producing tapes of reinforcing fibers that have undergone a        cracking operation and tapes of thermoplastic matrix fibers that        have undergone a cracking or chopping operation;    -   in producing the intimate mixture of tapes of these two        components with n₁ reinforcing fiber tapes and n′₁ matrix fiber        tapes, by pulling on the various parallel tapes and making them        pass through a system of needles in order to parallelize the        fibers while at the same time mixing them;    -   in successively carrying out this same operation using the        multicomponent tapes resulting each time from the previous        operation;    -   in continuously taking an assembly of parallel tapes, which        consists of an intimate mixture of reinforcing fibers and matrix        fibers obtained during the preceding operations, into a chamber        of heated to a temperature more than several tens of ° C. above        the melting point of the constituent material of the        thermoplastic matrix; and    -   passing the sheet between cooled rolls, which lower the        temperature of the sheet to below the solidification temperature        of the polymer and which exert pressure on the sheet in order to        form a preform consisting of mutually parallel reinforcing        fibers embedded in a solid thermoplastic polymer matrix.

Advantageously, before the sheet is introduced into the heated chamber,this process consists in continuously depositing on the sheet a tape oryarn of composition identical to the mixed tapes forming thelongitudinal direction that form, for example, zig-zags in a directionperpendicular to the direction of the reinforcing fibers.

In any event, the invention will be clearly understood with the aid ofthe description that follows, with reference to the appended schematicdrawing showing, by way of nonlimiting example, one embodiment of anapparatus for obtaining this sheet, together with illustrations of thefibrous mass:

FIG. 1 is a side view of a mixing machine;

FIG. 2 is a partial top view;

FIG. 3 is a side view of a sheet preconsolidation machine; and

FIGS. 4 and 5 are two views of the mixing of the constituent fibers ofthe sheet during manufacture of this sheet and pre-preconsolidation,respectively.

In practice, reinforcing fiber tapes and matrix fiber tapes areobtained, in the case of the reinforcing fibers, by a fiber cracking andmixing operation and, in the case of the matrix fibers, by a crackingoperation or a chopping operation.

The various reinforcing fiber tapes and thermoplastic matrix fiber tapesare deposited in pots 2, 3 respectively, placed beneath a table 4. Thetapes leaving the pots pass through openings 5 made in the table, theopenings 5 for the various pots being transversely offset with respectto one another, and some of the pots being longitudinally offsetrelative to one another. Each tape, after having passed through thetable 4, passes over a motor-driven roller 6 and is carried alonglongitudinally to the table, all the tapes being parallel to oneanother. The various tapes pass between two drive rolls 7 beforeentering a drawing head 8. This drawing head 8 comprises two opposedcombs 9 undergoing a square displacement movement, that is to say aforward movement, in which the tines of the combs are engaged in thetapes and are displaced in the same direction as the latter, followed bya comb retraction and return movement into a rear position some distancefrom the tapes, before again penetrating the latter. Placed downstreamof the drawing head 8 are forwarding rolls 10 which rotate more quicklythan the drive rolls 7 and impart a greater movement than thetranslational movement of the combs 9. The combination of the componentsimpose parallelism on the fibers relative to one another, and mixing ofthe reinforcing fibers with the matrix fibers. A mixed tape 12 iscollected in a collecting pot 13. It is followed in succession byseveral mixing operations of this type, taking, for each mixingoperation, following the first one, a series of mixed tapes obtainedfrom the immediately preceding mixing operation.

Thus:

-   -   if the first machine mixes n₁ reinforcing fiber tapes with n′₁        matrix fiber tapes,    -   the second machine mixes n₂ bi-component tapes coming from        machine No. 1,    -   the third machine mixes n₃ bi-component tapes coming from        machine No. 2,    -   and so on, until the m-th machine, which mixes nm bi-component        tapes coming from machine No. m-1.

The mixing is considered as valid if the product:n×n ₂ ×n ₃ . . . nm>300=T

(T: number of traces) with the condition:n=n ₁ if n ₁ <n′n=n′ ₁ if n′ ₁<n ₁.

The intimacy of mixing is an essential condition for obtaining apreconsolidated sheet in the shortest possible time interval.

FIGS. 4 and 5 show two schematic cross-sectional views of the fibrousmass, respectively after mixing, that is to say after the operation thathas just been described, and after the liberation of the thermoplasticmatrix fibers. In these figures, and especially FIG. 4, the reinforcingfibers are denoted by the reference 14 and the matrix fibers by thereference 15. In FIG. 5, the matrix fibers are no longer portrayed,since they have melted.

Proper distribution of the reinforcing fibers and matrix fibers makes itpossible to obtain a preconsolidated sheet in a very short time.

To be precise, the impregnation time is given by Darcy's law

$t_{1} = \frac{\eta\;{x^{2}\left( {1 - {vf}} \right)}}{k\left( {{Pe} - {Px}} \right)}$

-   -   t₁: impregnation time (seconds)    -   η: dynamic viscosity of the polymer melt (Pa·s)    -   x: mean distance between matrix fibers (m)    -   vf: decimal volume fraction of matrix    -   k: permeability of the fibrous binary mixture (m²)    -   Pe: external pressure applied to the preform (Pa)    -   Pc: capillary pressure (Pa).

We have been able to demonstrate the very strong dependence of theimpregnation time t_(i) compared with the mean distance x of the matrixfibers.

This distance x varies inversely with the number of traces T, reaching,however, an incompressible minimum value.

FIG. 3 shows a machine for continuously preconsolidating aunidirectional sheet. This machine has the same overall structure as themachine shown in FIGS. 1 and 2, so that the same elements are denoted bythe same references as previously. In this machine, all the pots 2, 3contain mixed tapes obtained from the previous mixing operation. Afterhaving passed between two stainless steel drive rolls, the sheetpenetrates a chamber 16 heated to a temperature above the melting pointof the polymer, this temperature increment possibly being around 60° C.The sheeting may be produced by infrared, of moderate wavelengths, inorder to ensure maximum energy absorption in the fibrous mass. Theheated chamber 16 is sealed and reflective. A perforated stainless steelconveyor 17 supports the fibrous sheet during the polymerheating/melting operation. The speed of advance of the sheet isidentical to the speed of advance of the sheet on the feed table.

Placed downstream of the chamber 16 are two series of calandering rolls18 which compress the sheet, lowering the temperature to below thesolidification point of the polymer. These metal rolls 18 are coatedwith polytetrafluoroethylene for those at the bottom and fitted with asilicone rubber or fluorinated rubber for those at the top. The variousrolls are cooled, for example by a water circulation system that can bemodulated in terms of flow rate. The pressure exerted by the rolls onthe preform must be greater than 10 bar (1 bar=101 300 Pa).

The preconsolidated sheet is then wound up on a receiving device 19 whenthe sheet thicknesses are less than or equal to 0.5 mm, or is cut intopanels with a length, for example, of 2 m in the case of thicknessesgreater than 0.5 mm.

The sheets thus obtained consist of mutually parallel reinforcing fibersembedded in a solid thermoplastic polymer matrix.

It is then possible at a manufacturer's premises to immediately cutblanks for the purpose of carrying out thermoforming operations. Theorientation of the fibers may be chosen according to the direction ofthe stresses, by superposing several cut blanks. A finished product maybe obtained as follows:

-   -   cutting of the blanks of unidirectional sheets;    -   heating by infrared;    -   deposition in a cold mold;    -   stamping;    -   removal.

The discontinuity of the reinforcing fibers induces a degree of freedomwhich makes it possible to obtain deep-drawn parts without breaking thearray of fibers, with a short production cycle time of between 1 and 3minutes for example.

An illustrative example of a composite unidirectional sheet according tothe invention is given below:

-   -   composition:        -   69% of craced carbon fibers with a mean length of 80 mm and            a strength of between 190 and 700 GPa,        -   31% of chopped nylon-12 polyamide fibers of 80 mm average            length;    -   First mixing machine:        -   23 carbon tapes,        -   6 polyamide tapes;    -   Second mixing machine:        -   12 carbon+polyamide mixed tapes;    -   Third mixing and consolidation machine:        -   30 carbon+polyamide mixed tapes;    -   Optimum mixing conditions:        n=n ₁=6 n ₁ <n ₂ with n ₂=23        T=6×12×30=2 160>300;    -   mass per unit area of the unidirectional sheet before and after        preconsolidation:        -   width: 0.6 m        -   mass per unit length of the tapes: 14 g/m        -   mass per unit area:

${\frac{14 \times 30}{0.6} = {700\mspace{14mu} g\text{/}m^{2}}};$

-   -   Thickness after consolidation:        -   density of the composite:            0.44×1.01+0.56×1.78=1.45 g/cm³        -   [lacuna]

${\frac{700}{1.45 \times 10^{4}} = {0.045\mspace{14mu}{cm}}};$

-   -   Heating temperature:        -   melting point of the polyamide: 78° C.        -   oven temperature: 178+60=238° C.

As is apparent from the foregoing, the invention greatly improves theexisting technique by providing a composite unidirectional sheet, whichis simple in structure and inexpensive and rapid to process, allowing itto be used in many industrial applications.

As goes without saying, the invention is not restricted to only the oneembodiment of this sheet, described above by way of example, but on thecontrary it encompasses all variants thereof. Thus, in particular, thereinforcing fibers could be of several different types without therebydeparting from the scope of the invention.

1. A unidirectional sheet made of a composite, wherein theunidirectional sheet comprises an intimate mixture of mixed tapes,wherein the mixed tapes comprise long discontinuous and mutuallyparallel reinforcing fibers obtained by cracking and long thermoplasticmatrix fibers, and wherein the reinforcing fibers and thermoplasticmatrix fibers are heated such that the reinforcing fibers are embeddedin a solid thermoplastic polymer matrix formed of melted thermoplasticmatrix fibers.
 2. The unidirectional sheet as claimed in claim 1,wherein the average length of the reinforcing fibers is about 80 mm witha variation in the lengths of individual ones of the reinforcing fibersbeing less than or equal to 35% with respect to this average length. 3.The unidirectional sheet as claimed in claim 1, wherein the number oftraces of reinforcing fibers in the final mixture is greater than 300.4. The unidirectional sheet as claimed in claim 1, wherein theconstituent reinforcing fibers of a sheet are chosen from at least oneof the families of following materials: carbon, coming from PAN, pitchor rayon precursors with the criterion: 190 GPa<E<700 Gpa (E being thetensile modulus); glass of the E, R, S or D type; para-aramid with thecriterion: 60 Gpa<E<140 Gpa (E being the tensile modulus);polybenzimidazole (PBO) of the AS-H-M type with the criterion: E>190 Gpa(E being the tensile modulus).
 5. The unidirectional sheet as claimed inclaim 1, wherein the constituent polymer matrix of a sheet is chosenfrom at least one of the families of the following heat-stablematerials: polyphenylenesulfide (PPS), polyetherimide (PEI),polyethersulfone (PES), polyetheretherketone (PEEK) and polyetherketone(PEK).
 6. The unidirectional sheet as claimed in claim 1, wherein theconstituent polymer matrix of a sheet is chosen from at least one of thefamilies of the following standard thermoplastics: polyetherterephthalate (PET), polyamide (PA), polypropylene (PP), polycarbonate(PC), polybutadiene terephthalate (PBT) and polyethylene (PE).
 7. Aprocess for manufacturing a unidirectional sheet made of a composite asclaimed in claim 1, comprising: producing tapes of reinforcing fibersthat have undergone a cracking operation and tapes of thermoplasticmatrix fibers that have undergone a cracking or chopping operation;producing the intimate mixture of tapes of these two components with n₁reinforcing fiber tapes and n′₁ matrix fiber tapes, by pulling on thevarious parallel tapes and making them pass through a system of needlesin order to parallelize the fibers while at the same time mixing them;successively carrying out this same operation of producing the intimatemixture of tapes using the multicomponent tapes resulting each time fromthe previous operation of producing tapes; continuously taking anassembly of parallel tapes, which comprises an intimate mixture ofreinforcing fibers and matrix fibers obtained during the precedingoperations, into a chamber of heated to a temperature more than severaltens of ° C. above the melting point of the constituent material of thethermoplastic matrix; and passing the sheet between cooled rolls, whichlower the temperature of the sheet to below the solidificationtemperature of the polymer and which exert pressure on the sheet inorder to form a preform consisting of mutually parallel reinforcingfibers embedded in a solid thermoplastic polymer matrix.
 8. The processas claimed in claim 7, wherein before the sheet is introduced into theheated chamber, the process further comprises continuously depositing onthe sheet a tape or yarn of composition identical to the mixed tapesforming the longitudinal direction that form zig-zags in a directionperpendicular to the direction of the reinforcing fibers.